Hands free telephones, also known as speaker phones, have a problem with echo feedback. Examples of speaker phones are: mobile cellular telephones which may be used in automobiles; teleconferencing systems; and desk top telephones. The sound from a speaker propagates into the room including the speaker phone and echoes return to the microphone. This acoustic feedback disturbs the normal speaking. Acoustic echo cancellation systems sample some of the incoming signal and construct an estimate of the echo. The echo estimate is subtracted from the microphone signal to produce an echo reduced signal to sent to the far-end unit.
A transversal adaptive filter is typically used in acoustic echo cancellation systems. The maximum echo delay determines the size of the transversal filter. Acoustic echo cancellation systems need to be adaptive because the manufacturer of the speaker phone typically does not know the exact acoustic environment where the speaker phone is used. In the case of mobile cellular telephones, the user will change the location of use and thereby change the acoustic environment. Even for speaker phones used in fixed locations there will be a wide variation in the acoustic environment between possible use locations.